1. Field of the Invention
The present invention relates to flat display screens. The present invention applies, more specifically, to so-called cathodoluminescence screens, the anode of which supports phosphor elements, separated from one another by insulating areas, and likely to be energized by electron bombardment. This electron bombardment can come from microtips, from layers of low extraction potential, or from a thermo-ionic source.
The present invention more specifically relates to the definition of an internal space, generally in vacuum conditions, wherein flow the electrons emitted by the screen cathode.
To simplify the present description, only color microtip screens will be considered hereafter, but it should be noted that the present invention relates, generally, to the various above-mentioned types of screens and the like.
2. Discussion of the Related Art
FIG. 1 shows the structure of a conventional flat color microtip screen, essentially formed of a cathode 1 with microtips 2 and of a grid 3 provided with holes 4 corresponding to the locations of microtips 2. Cathode 1 is placed facing a cathodoluminescent anode 5, a glass substrate 6 of which forms the screen surface.
The operating principle and an example of embodiment of a microtip screen are described, in particular, in U.S. Pat. No. 4,940,916 of the Commissariat a l'Energie Atomique.
Cathode 1 is organized in columns and is formed, on a glass substrate 10, of cathode conductors organized in meshes from a conductive layer. Microtips 2 are made on a resistive layer 11 deposited on the cathode conductors and are arranged within the meshes defined by the cathode conductors. FIG. 1 partially shows the inside of a mesh and the cathode conductors do not appear on the drawing. Cathode 1 is associated with grid 3 organized in lines. The intersection of a line of grid 3 and of a column of cathode 1 defines a pixel.
This device uses the electric field created between cathode 1 and grid 3 to extract electrons from microtips 2. These electrons are then attracted by phosphor elements 7 of anode 5 if these elements are properly biased. In the case of a color screen, anode 5 is provided with alternate strips of phosphor elements 7r, 7g, 7b, each corresponding to a color (Red, Green, Blue). The strips are parallel to the cathode columns and are separated from one another by an insulator 8, generally silicon oxide (SiO.sub.2). Phosphor elements 7 are deposited on electrodes 9, formed of corresponding strips of a transparent conductive layer such as indium and tin oxide (ITO).
The assembly of both substrates or plates 6 and 10 respectively supporting anode 5 and cathode 1 is performed by creating a vacuum space 12 of circulation of the electrons emitted by cathode 1. The distance between cathode 1 and anode 5 must be constant so that the screen brightness is regular over its entire surface. Spacers, generally constituted by balls, generally made of glass, of a diameter corresponding to the desired distance between electrodes, are regularly distributed on one of the plates, before the plates are assembled together.
The distance between electrodes, defined by the ball diameter, conventionally is on the order of 200 .mu.m while the space between two cathode conductors corresponding to different columns is of a given value included between approximately 10 and 100 .mu.m and the pixels pitch is of a given value included between approximately 50 and 300 .mu.m.
A problem which then arises is to maintain the balls in their position until the screen is assembled. Indeed, if balls are, during the assembly, in active areas of the screen, they form obstacles to the path of the electrons emitted by microtips 2 towards phosphors 7, which creates shaded areas. To solve this problem, the balls are generally glued to the cathode before assembly.
Patent FR-A-2727242 describes an example of a technique for gluing balls on the cathode. This technique consists of using an application plate, of the screen dimension, provided with circular notches of reception of balls to be glued. The bottom of the notches is pierced to communicate with a suction chamber. Balls placed in bulk in an appropriate container are first sucked in. Then, while maintaining the suction, the balls are put in contact with a plate coated with glue, to deposit a touch of glue on each ball. The cathode-grid plate is then applied on the application plate. Finally, the suction is cut off and the cath-ode-grid plate is moved away from the suction plate. The balls then remain glued on the cathode-grid plate at the locations defined by the notches of the application plate.
Another known gluing technique consists of using, instead of a pierced application plate, a hollow needle to take, spread glue on, and position the balls. This technique is described in U.S. Pat. No. 5,558,732.
A disadvantage of these techniques is that the glue causes a pollution of the surface of the cathode-grid and obliges to perform a vacuum degassing thermal processing.
Another disadvantage is that, as long as the glue is not dry, the balls are still likely to slightly move. Further, during the degassing, a large amount of the glue is evaporated and the balls then risk to move in case of an abrupt motion. Now, the dimensional constraints indicated hereabove lead to the fact that a shifting, even slight, of the ball positions, can have prejudicial consequences by creating shaded areas.
Another disadvantage of these techniques is that they require to position the balls on the cathode plate while the cathode receives the electron emission microtips, which elements are particularly sensitive to degassings. Indeed, it is not possible to glue the balls on the anode since the deposited phosphor elements, on the anode side, would be damaged by the thermal degassing processing.